The biology of human parainfluenza virus type 3 (para 3) is to be elucidated. The research is being approached from the perspective of a finding that when para 3 is propagated by undiluted passage in vitro, its capacity to cause syncytial cytopathic effects (CPE) in cell monolayers is inhibited. In the absence of CPE, the monolayers are not destroyed and persistent infection, an "endosymbiosis" of virus and cell culture in which virus is transmitted from one cell generation to the next without destruction of the culture, is readily established. Our research has demonstrated that this suppression of viral CPE is caused by the production of a syncytium inhibitor. This is a small virus protein. The nature of this protein, its relationship to the other peptides of para 3, and the importance of these phenomena and of persistent infection for in vivo infections caused by para 3 are being clarified. The mechanisms responsible for the maintenance, as well as the initiation, of persistent infection are also being studied. This knowledge will help to define the pathophysiology of this important respiratory pathogen by clarifying the vagaries of its replicative cycle in vitro and in vivo.